Processing method

ABSTRACT

A processing method includes disposing a workpiece having a processed surface in a processing solution, disposing a photocatalyst film in the processing solution opposite the processed surface, irradiating the photocatalyst film with a light, so as to generate active species from the processing solution by a photocatalytic action of the photocatalyst film, controlling a diffusion distance of the active species in the processing solution by a radical scavenger added to the processing solution, and chemically reacting the active species with surface atoms of the processed surface and generating a chemical compound to be eluted in the processing solution, so as to process the workpiece.

The present application is based on Japanese patent application No. 2010-176911 filed on Aug. 6, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a processing method. In particular, this invention relates to a processing method for processing a surface of a workpiece.

2. Description of the Related Art

Conventionally, as a method of processing a surface of a workpiece such as a semiconductor wafer and the like, a catalyst-aided chemical processing method is known, the processing method including a step of disposing a workpiece having a processed surface in a processing solution containing an oxidizing agent, a step of bringing a solid catalyst for decomposing the oxidizing agent into contact with or in close proximity to the processed surface, a step of generating active species having oxidizing force from the processing solution by using a catalyst action of the solid catalyst, so as to chemically react the active species with surface atoms of the processed surface and generate a chemical compound and a step of removing the chemical compound generated, and the processing method processing the processed surface by using either one or a combination of not less than two of a step of irradiating the processed surface with a light, a step of applying a voltage between the processed surface and the solid catalyst, and a step of controlling the temperatures of the solid catalyst, the workpiece and/or the processing solution, during the processing of the processed surface. The processing method is disclosed in, for example, JP-A-2008-136983.

The catalyst-aided chemical processing method disclosed in JP-A-2008-136983 chemically processes a processed surface of a workpiece, so that the processed surface can be processed so as to have a high-accuracy surface.

SUMMARY OF THE INVENTION

However, the catalyst-aided chemical processing method disclosed in JP-A-2008-136983 decomposes an oxidizing agent in a surface of a solid catalyst that becomes a processing reference surface and generates active species that are used for a chemical reaction with the processed surface, so that the active species do not exist in any place except for places on or in proximity to the surface of the solid catalyst. Consequently, the flatness of surface of the solid catalyst is transferred onto the surface of the workpiece, and it is difficult to allow the processed surface to surpass the surface of the solid catalyst in the flatness.

Therefore, it is an object of the invention to solve the above-mentioned problem and provide a processing method that is capable of forming a processed surface of a workpiece having a high flatness property and not having a processing-degenerated layer by processing the processed surface of the workpiece.

(1) According to one embodiment of the invention, a processing method comprises:

disposing a workpiece having a processed surface in a processing solution,

disposing a photocatalyst film in the processing solution opposite the processed surface,

irradiating the photocatalyst film with a light, so as to generate active species from the processing solution by a photocatalytic action of the photocatalyst film,

controlling a diffusion distance of the active species in the processing solution by a radical scavenger added to the processing solution, and

chemically reacting the active species with surface atoms of the processed surface and generating a chemical compound to be eluted in the processing solution, so as to process the workpiece.

In the above embodiment (1) of the invention, the following modifications and changes can be made.

(i) The workpiece is processed by controlling a temperature of at least one member selected from the group consisting of the photocatalyst film, the workpiece and the processing solution.

(ii) The radical scavenger comprises a protic organic compound.

(iii) The protic organic compound comprises one of methanol, ethanol, propanol and butanol, or a mixture liquid of not less than two selected therefrom.

(iv) The photocatalyst film comprises a film of TiO₂, and the TiO₂ comprises an anatase-type crystal or a rutile-type crystal, or a mixed crystal of the anatase-type crystal and the rutile-type crystal.

(v) The light has a wavelength of not more than 420 nm.

(vi) The photocatalyst film is disposed on a substrate formed of quartz or glass.

(vii) The light is irradiated from a side of the substrate toward the photocatalyst film, so as to generate active species.

(viii) The workpiece disposed in the processing solution comprises at least one material selected from the group consisting of SiC, GaN, sapphire, ruby and diamond.

Points of the Invention

According to one embodiment of the invention, a processing method is conducted such that the surface of a workpiece is sequentially processed in the increasing order of distance (i.e., from nearest to farthest) from a photocatalyst film that is disposed opposite the processed surface (i.e., the surface subjected to the processing) of the workpiece. Thereby, a substrate with a good flatness property can be produced. In other words, the processed surface of the workpiece can be processed with anisotropy to provide a substrate with a good flatness property. For example, the diffusion distance of an active species in a processing solution can be controlled by adding a radical scavenger into the processing solution, and the controlled diffusion distance of the active species allows an oxidation reaction to be sequentially conducted in the increasing order of distance (i.e., from nearest to farthest) of the processed surface from the photocatalyst film.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments according to the invention will be explained below referring to the drawings, wherein:

FIG. 1A is a conceptual view schematically showing a processing method according to one embodiment of the invention;

FIG. 1B is a conceptual view schematically showing a case that a workpiece is processed by the processing method according to the embodiment of the invention;

FIG. 2 is an explanatory view schematically showing an oxidation-reduction process of a photocatalytic reaction that is a process principle of the processing method according to the embodiment of the invention;

FIG. 3 is a conceptual view schematically showing a processing method according to Example 1;

FIG. 4 is a conceptual view schematically showing a processing method according to Example 2;

FIG. 5 is a graph showing an oxygen atom concentration in the respective surfaces of SiC substrates of Comparative Example 2 corresponding to a case before the processing method of the invention is applied thereto, Example 1 corresponding to a case that an aqueous solution of water and ethanol is used as a processing solution and Comparative Example 1 corresponding to a case that only water is used as a processing solution; and

FIG. 6 is a graph showing a root-mean-square surface roughness (Rms) of the processed surface of the SiC substrate relative to the respective reaction times according to Example 1, Example 2 and Comparative Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments

Outline of Processing Method

FIG. 1A is a conceptual view schematically showing a processing method according to one embodiment of the invention. In addition, FIG. 2 is an explanatory view schematically showing an oxidation-reduction process of a photocatalytic reaction that is a processing principle of the processing method according to the embodiment of the invention.

A processing method according to the embodiment is a photocatalytic reaction type chemical processing method that processes a workpiece 20 by using active species 40 generated from a processing solution 30 due to a light irradiation.

An outline of the processing method according to the embodiment will be explained with reference to FIG. 1A. The processing method according to the embodiment includes a workpiece disposition step of disposing a workpiece 20 having a surface 20 a to be processed in a processing solution 30, a photocatalyst film disposition step of disposing a photocatalyst film 12 in the processing solution 30 opposite the surface 20 a to be processed, a active species generation step of irradiating the photocatalyst film 12 with a light 60, so as to generate active species 40 being radicals from the processing solution 30 by a photocatalytic action of the photocatalyst film 12, and a processing step of chemically reacting the active species 40 with surface atoms 22 of the surface 20 a to be processed, generating a chemical compound 50 to be eluted in the processing solution 30 and eluting the chemical compound 50 in the processing solution 30, so as to process the workpiece 20.

Further, the processing step can also have a feature that the workpiece 20 is processed by controlling a temperature of at least one member selected from the group consisting of the photocatalyst film 12, the workpiece 20 and the processing solution 30. In the processing step, the temperature of at least one member selected from the group consisting of the photocatalyst film 12, the workpiece 20 and the processing solution 30 is controlled, so that a chemical reaction speed between the workpiece 20 and the active species 40 can be controlled in an increasing direction or a decreasing direction. For example, if the temperature is raised, the chemical reaction speed is increased, and if the temperature is lowered, the chemical reaction speed is decreased

In the embodiment, the photocatalyst film 12 is formed on a front surface 10 b of a substrate 10 that transmits a light 60. Consequently, the light 60 that enters a rear surface 10 a of the substrate 10 transmits the substrate 10 and enters the photocatalyst film 12 formed on the front surface 10 b. In addition, the processing solution 30 contains a radical scavenger 42 capable of capturing the active species 40 being radicals. The radical scavenger 42 captures the active species 40 and becomes a chemical compound 52. With regard to the active species 40, some are captured by the radical scavenger 42, and some are not be captured, react with surface atoms 22 of the surface 20 a to be processed and generate a chemical compound 50.

Details of Processing Method

First, as shown in FIG. 1A, in the processing solution 30 containing the radical scavenger 42, the photocatalyst film 12 is brought into contact with or in close proximity to the workpiece 20.

Further, in case that the photocatalyst film 12 is brought in close proximity to the workpiece 20, a distance between the front surface 12 a of the photocatalyst film 12 and the workpiece 20 is within a range of the maximum diffusion distance of the active species 40 in the processing solution 30 that is determined by life-span of the active species 40 generated from the processing solution 30.

For example, in case that the processing solution 30 is composed of only water, the active species 40 generated are hydroxyl radicals that have strong oxidation power, and the maximum diffusion distance in this case is approximately 1 μm. In case that the processing solution 30 is composed of water and the radical scavenger 42 added to water, the hydroxyl radical diffused in the processing solution 30 reacts with the radical scavenger 42 and becomes the chemical compound 52, so that the maximum diffusion distance becomes shorter than that of the above-mentioned case, namely becomes less than 1 μm. Consequently, it is only necessary to control the distance brought in close proximity to be less than 1 μm.

Next, the light 60 is irradiated from a side of the rear surface 10 a of the substrate 10, and then the light 60 that passes through the substrate 10 reaches the photocatalyst film 12. When the photocatalyst film 12 is irradiated with the light 60, the active species 40 generated from the processing solution 30 by a photocatalytic action of the photocatalyst film 12 adhere to the front surface 12 a of the photocatalyst film 12. And, the active species 40 generated are diffused in the processing solution 30 toward the surface 20 a to be processed. A part of the active species 40 chemically reacts with the radical scavenger 42 before it reaches the surface 20 a to be processed of the workpiece 20, so as to generate the chemical compound 52. Consequently, as the diffusion distance is increased in the processing solution 30, probability that a part of the plurality of active species 40 generated from the processing solution 30 is deactivated until it reaches the surface 20 a to be processed is increased. Due to this, the workpiece 20 is processed sequentially from the part thereof that has the shortest distance from the photocatalyst film 12 to the workpiece 20 (for example, in case that there are concavity and convexity on the surface 20 a to be processed, from the fore-end part of the convexity). As just described, the active species 40 that reach the surface 20 a to be processed before they react with the radical scavenger 42 chemically react with the surface atoms 22 of the surface 20 a to be processed, so as to generate the chemical compound 50 to be eluted in the processing solution 30. Subsequently, the chemical compound 50 generated is eluted and diffused from the surface 20 a to be processed into the processing solution 30.

Due to this, the surface 20 a to be processed of the workpiece 20 is processed. In particular, for example, as shown in FIG. 1B, the surface of the workpiece 20 is processed and flat surfaces 20 b are exposed in the processing solution 30.

Substrate 10

The substrate 10 according to the embodiment is formed of a transparent material that transmits the light 60. In particular, in case that the light 60 is an ultraviolet light, the substrate 10 can be formed of a material that transmits the ultraviolet light. For example, as the substrate 10, a glass substrate, a quartz substrate, a substrate formed of a synthetic resin such as an acrylic resin and the like can be used. The substrate 10 is formed of the transparent material that transmits the light 60, so that the photocatalyst film 12 can be irradiated with the light 60 from a side of the rear surface 10 a of the substrate 10. Further, in case that the substrate 10 is formed of the synthetic resin, the substrate 10 is used, the substrate 10 having a transmittance of the light 60 of the degree that the synthetic resin is hard to be deteriorated against long-term use and simultaneously having the front surface 10 b that has a flatness property not less than the flatness required for the surface 20 a to be processed of the workpiece 20.

Photocatalyst Film 12

The photocatalyst film 12 according to the embodiment is formed on the front surface 10 b of the substrate 10 opposite the surface 20 a to be processed of the workpiece 20. And, the photocatalyst film 12 is formed of a film composed of a photocatalyst or a film including the photocatalyst. As the photocatalyst constituting the photocatalyst film 12, at least one chemical compound selected from the group consisting of the metal oxides such as TiO₂, KTaO₃, SrTiO₃, ZrO₂, NbO₃, ZnO, WO₃, SnO₂ and the like that are metal oxides which have energy of not less than approximately 2.8 eV in the superior end of the valence band can be used. In addition, these chemical compounds can be doped with impurities. For example, the photocatalyst film 12 can be also formed of a nitrogen doped photocatalyst (for example, N-doped TiO₂) that is doped with nitrogen (N).

Here, in case that TiO₂ is used as the photocatalyst, it is preferred to use TiO₂ that has a crystal structure of an anatase-type. Further, TiO₂ of a rutile-type crystal, or a mixed crystal of TiO₂ of an anatase-type crystal and TiO₂ of a rutile-type crystal can be also used.

Method of Manufacturing the Photocatalyst Film 12

The photocatalyst film 12 according to the embodiment can be manufactured by using a sputtering method, a vapor-deposition method, a molecular beam epitaxy method (a MBE method), a laser ablation method, an ion plating method, a thermal CVD method, a plasma CVD method, a metal organic chemical vapor deposition method (a MOCVD method), a liquid phase epitaxy method, an aerosol deposition method (an AD method), a Langmuir-Blodgett method (a LB method), a sol-gel method, a plating method, a coating method or the like. Here, in the embodiment, it is preferred to use the sputtering method from the standpoint that the film formation is easily controlled and the like.

In case that the photocatalyst film 12 is manufactured by using the sputtering method, it can be manufactured as follows. For example, the sputtering is carried out by using a target formed of TiO₂ under an Ar atmosphere, so that the photocatalyst film 12 that is formed by being directly deposited as TiO₂ can be formed on the substrate 10. In addition, the sputtering is carried out by using a target formed of Ti under an O₂ and Ar mixture atmosphere (hereinafter, may be referred to as O₂/Ar atmosphere), so that the photocatalyst film 12 composed of TiO₂ formed by that Ti and O₂ in the atmosphere are reacted can be formed on the substrate 10. Further, as a sputtering device for carrying out the sputtering method, a direct current sputtering device, a high-frequency sputtering device, a magnetron sputtering device, an ion beam sputtering device, an electron cyclotron resonance (ECR) sputtering device and the like can be used.

In addition, in case that the photocatalyst film 12 is formed by using the sputtering method, for the purpose of preventing mean free path of plasma such as Ar and the like from being increased so as to reduce a damage to the photocatalyst film 12 during the film formation, it is preferable that an output of the plasma is set to not more than 400 W and total pressure of gas in a chamber is set to not less than 1.0 Pa.

Further, for the purpose of increasing a generation amount of the active species 40 that chemically react with the surface atoms 22 of the surface 20 a to be processed so as to carry out the processing of the workpiece 20 at a sufficient speed, it is preferable that the photocatalyst film 12 is formed to have a film thickness of not less than 150 nm that is a thickness capable of sufficiently absorbing the light 60. In addition, it is more preferable that the film thickness of the photocatalyst film 12 is not less than 200 nm. Furthermore, in order that an amount of the active species 40 generated in accordance with an amount of the light that is irradiated from a side of the rear surface 10 a of the substrate 10 toward the photocatalyst film 12 and reaches the photocatalyst film 12 becomes an amount sufficient for the processing of the workpiece 20, it is preferable that the film thickness of the photocatalyst film 12 is not more than 1 μm.

In addition, as the light 60 in the embodiment, a light that has energy of not less than a band gap energy of the photocatalyst constituting the photocatalyst film 12 is used. For example, since the band gap energy of TiO₂ is 3.0 eV, TiO₂ fulfills a photocatalytic function to a light having a wavelength of not more than 420 nm. Consequently, in case that TiO₂ is used as the photocatalyst, as the light 60, a light having a wavelength of preferably not less than 200 nm and not more than 420 nm, and more preferably not less than 200 nm and not more than 400 nm is used. Further, in case that the photocatalyst constituting the photocatalyst film 12 fulfills a photocatalytic function to a visible light, as the light 60, the visible light can be also used.

Workpiece 20

The workpiece 20 according to the embodiment is formed of, for example, crystal materials such as a semiconductor material, an oxide material and the like that are used for an electronic device such as a power device, a light emitting device and the like. In particular, the workpiece 20 is a substrate formed of a crystal material such as SiC, GaN, sapphire, ruby, diamond or the like that has poor processability.

Radical Scavenger 42

As the radical scavenger 42 according to the embodiment, a protic organic compound can be used. As the radical scavenger 42 of the protic organic compound, for example, any one of methanol, ethanol, propanol and butanol can be used. In addition, as the radical scavenger 42, a mixture liquid prepared by selecting at least two from methanol, ethanol, propanol and butanol and mixing the selected at least two protic organic compounds can be also used.

Process Principle of Processing Method: Generation Reaction of Active Species

FIG. 2 is an explanatory view schematically showing an outline of oxidation-reduction process of a photocatalytic reaction that is a process principle of the processing method according to the embodiment.

With reference to FIG. 2, a principle that the active species 40 are generated from the processing solution 30 (for example, water) in the place on or adjacent to the surface of the photocatalyst film 12 will be explained. Here, as an example, a case that a TiO₂ is used as the photocatalyst will be explained. First, TiO₂ is irradiated with a light that has energy of not less than the band gap energy of TiO₂ and a wavelength of not more than 420 nm, so that in accordance with Reaction formula (1) described below, electrons existing in the valence band are excited to the conduction band so as to generate positive holes in the valence band, and simultaneously excited electrons are generated in the conduction band so as to generate a pair of positive hole and electron. Further, in the Reaction formula (1), “UV” represents an ultraviolet light for short.

TiO₂+light (UV)→h ⁺ +e ⁻  (Reaction formula (1))

The positive holes extract electrons (e⁻) from hydroxyl ions (OH⁻) generated due to ionization of water (H₂O) so as to generate hydroxyl radicals (.OH) in accordance with reactions shown in Reaction formula (2) and Reaction formula (3) described below.

H₂O→H⁺+OH⁻  (Reaction formula (2))

h ⁺+OH⁻→OH  (Reaction formula (3))

The hydroxyl radicals generated in accordance with the reaction shown in Reaction formula (3) have extremely strong oxidation power. Consequently, the hydroxyl radicals are able to react with chemically stable materials such as SiC, GaN, diamond and the like, and process the chemically stable materials.

On the other hand, the electrons excited move toward oxygen gas (dissolved oxygen) dissolved in the processing solution 30 in accordance with Reaction formula (4) described below so as to reduce the oxygen, unless specific substances (sacrificial reagents) that are readily oxidized are added into the processing solution 30. Further, it can be also adopted to enhance reaction efficiency by adding the sacrificial reagents instead of the dissolved oxygen into the processing solution 30.

O₂+e⁻→O₂ ⁻  (Reaction formula (4))

Process Principle of Processing Method: Reaction of Active Species and Workpiece and Processing Step [in Case of SiC]

Next, a processing step of SiC as the workpiece 20 in case that the workpiece is SiC will be explained. First, it can be considered that the surface of SiC is oxidized due to the active species 40 (as an example, hydroxyl radicals in case that the processing solution 30 is water) generated from the processing solution 30 by a light irradiation to the photocatalyst film 12 in accordance with Reaction formula (5) described below.

SiC+4.OH+O₂→SiO₂+CO₂+2H₂O  (Reaction formula (5))

Here, in case that the radical scavenger 42 is added into the processing solution 30, the hydroxyl radicals as the active species 40 react with the radical scavenger 42 before they reach the surface 20 a to be processed that is the surface of the workpiece 20, so that they are deactivated. Consequently, the diffusion distance of the hydroxyl radicals in the processing solution 30 is determined by reaction speed between the hydroxyl radicals and the radical scavenger 42. As just described, by adding the radical scavenger 42 into the processing solution 30 and controlling the diffusion distance of the active species 40 in the processing solution 30, the diffusion distance can advance an oxidation reaction sequentially from the surface 20 a to be processed that is located at the nearest distance from the photocatalyst film 12 (for example, TiO₂ thin film).

It can be considered that after the oxidation reaction of the surface of the workpiece 20, by applying a removal treatment of an oxide layer generated by the oxidation reaction to the surface 20 a to be processed, the oxidized regions of the surface 20 a to be processed are preferentially processed. Further, in case that the oxide layer generated by the oxidation reaction is SiO₂, hydrofluoric acid can be used for the removal treatment of the oxide layer. In this case, the oxidized regions of the surface 20 a to be processed are preferentially processed in accordance with Reaction formula (7) described below.

SiO₂+6HF→H₂SiF₆+2H₂O  (Reaction formula (6))

Process Principle of Processing Method: Reaction of Active Species and Workpiece and Processing Step [in Case of GaN]

In addition, a processing step of GaN as the workpiece 20 in case that the workpiece is GaN will be explained. First, it can be considered that the surface of GaN is oxidized due to the active species 40 (as an example, hydroxyl radicals in case that the processing solution 30 is water) generated from the processing solution 30 by a light irradiation to the photocatalyst film 12 in accordance with Reaction formula (7) described below.

2GaN+7.OH+7/4O₂→Ga₂O₃+2NO₂+7/2H₂O  (Reaction formula (7))

In this case, similarly to the explanation about the case that the workpiece 20 is SiC, by adding the radical scavenger 42 into the processing solution 30, the diffusion distance of hydroxyl radicals as the active species 40 can be controlled. And, it can be considered that after the oxidation reaction of the surface of GaN as the workpiece 20, by applying a removal treatment of an oxide layer generated by the oxidation reaction to the surface 20 a to be processed, the oxidized regions of the surface 20 a to be processed are preferentially processed. Further, in case that the oxide layer generated by the oxidation reaction is Ga₂O₃, sulfuric acid can be used for the removal treatment of the oxide layer. In this case, the oxidized regions of the surface 20 a to be processed are preferentially processed in accordance with Reaction formula (8) described below.

Ga₂O₃+3H₂SO₄→Ga₂(SO₄)₃+3H₂O  (Reaction formula (8))

Advantages of the Embodiment

The processing method according to the embodiment includes disposing the workpiece 20 in the processing solution 30 and simultaneously disposing the photocatalyst film 12 in the processing solution 30 opposite the surface 20 a to be processed, and then irradiating the photocatalyst film 12 with the light 60, so as to generate active species 40 from the processing solution 30, controlling a diffusion distance of the active species 40 in the processing solution 30 by the radical scavenger 42 added to the processing solution 30, and chemically reacting the active species 40 with the surface atoms 22 of the surface 20 a to be processed and generating the chemical compound 50 to be eluted in the processing solution 30, so as to process the workpiece 20, so that mechanical defects generated in the workpiece 20 when abrasive grain or abrasive compound is used are not generated. Due to this, in accordance with the processing method according to the embodiment, a workpiece not having a processing-degenerated layer (for example, a damaged layer generated on the surface of workpiece 20 when polishing is carried out by using the abrasive compound or the like) and having a surface with high accuracy (namely, a surface with a high flatness property) can be manufactured without generating crystal defects due to the processing.

In addition, the processing method according to the embodiment does not need the abrasive grain or abrasive compound to be used, so that for example, disposal of industrial waste such as disposal of used slurry required in a polishing method such as chemical mechanical polishing (CMP) method or the like is not needed. Consequently, the processing method according to the embodiment can reduce a cost of disposal, and does not discharge the industrial waste, so that the processing method is a preferable processing method from the viewpoint of conservation of environment.

Further, the processing method according to the embodiment has a step that the processing is carried out sequentially from the surface 20 a to be processed that is located at near distance from the photocatalyst film 12, so that a substrate having a good flatness property can be manufactured. In other words, the processing method according to the embodiment can process the surface of the workpiece 20 with anisotropic aspect, so that a substrate having a good flatness property can be manufactured.

Example 1

Hereinafter, Examples will be explained.

FIG. 3 is a conceptual view schematically showing an outline of processing method according to Example 1.

Fabrication of Photocatalyst Film 12

First, a quartz substrate 14 using quartz as a base material was disposed in a chamber of a high-frequency magnetron sputtering device. And, a TiO₂ thin film as the photocatalyst film 12 was formed on the surface of quartz substrate 14 under an Ar gas 100% atmosphere under the condition that an output of the plasma was 300 W, the substrate temperature was 300 degrees C., the total pressure was 3 Pa, the film formation time was 24 minutes. Further, as the target of high-frequency magnetron sputtering device, a target formed of a TiO₂ fired body was used.

Evaluation of Photocatalyst Film

Properties of the TiO₂ thin film formed on the surface of quartz substrate 14 were evaluated. The TiO₂ thin film had a film thickness of 200 nm. In addition, the TiO₂ thin film had a crystal structure of a mixed crystal of an anatase type crystal 76% and a rutile-type crystal 24%.

Photocatalytic Reaction Type Chemical Processing of SiC Substrate

In Example 1, a SiC substrate was used as the workpiece 20. In particular, the SiC substrate used in Example 1 is a SiC substrate of a single crystal, and an n-type 4H—SiC that has a diameter of 50 mm and has (0001) Si face inclined by 8 degrees in a [11-20] direction was used. In addition, an electrical resistivity of the SiC substrate was 0.017 Ωcm. Further, the SiC single crystal substrate of {0001} face has polarity, one is a Si face formed of which outmost face is composed of Si atoms and another is a C face formed of which outmost face is composed of C atoms. In Example 1, the Si face was used as the surface 20 a to be processed. In addition, in Example 1, the surface 20 a (Si face) to be processed before the processing is a surface to which the CMP treatment was applied after a mechanical mirror-polishing was applied thereto. Further, in case that the C face was used as the surface 20 a to be processed, although the case is somewhat different in a processing speed from the case that the Si face was used, a mechanism of advancement of the processing is similar to the case of the Si face.

In particular, the processing method according to Example 1 is as follows. First, the surface of SiC substrate was cleaned with a 10% HF aqueous solution. Next, after the SiC substrate was put into a glass beaker 70, the quartz substrate 14 on which the photocatalyst film 12 was formed was introduced. In this case, the processed surface of the SiC substrate and the photocatalyst film 12 were brought into contact with each other. Next, an aqueous solution 32 prepared by adding methanol as a radical scavenger to water was introduced into the glass beaker 70 as a processing solution. Here, as the aqueous solution 32, an aqueous solution prepared by adjusting the concentration of methanol to 50% (volume/volume %: v/v %) was used. In this case, the aqueous solution 32 was introduced into the glass beaker 70 so that the surface of aqueous solution 32 was located at an upper side than a contact surface (hereinafter, may be referred to as “interface”) of the photocatalyst film 12 and the processed surface of the SiC substrate (namely, a position that at least the contact surface exists in the aqueous solution 32). Due to this, the aqueous solution 32 entered the interface of the photocatalyst film 12 and the processed surface of the SiC substrate by capillarity so that an aqueous solution film layer 34 (hereinafter, may be referred to as “radical transport layer”) was formed.

Next, an ultraviolet light 62 having an ultraviolet light illuminance adjusted to 8 mW/cm² was irradiated from a side of rear surface 14 a of the quartz substrate 14 by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 in the aqueous solution film layer 34 was initiated. With regard to a reaction time of the photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 (namely, a reaction time of the active species and the workpiece), the processed surface was observed after every reaction for 1 hour and the reaction was carried out up to 5 hours as an accumulated time.

Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the SiC substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a 10% HF aqueous solution. And, the processed surface after the processing was observed by an atomic force microscope (AFM). After that, for further 1 hour, the photocatalytic reaction was similarly carried out, and the oxide film was removed and the processed surface was observed. This was repeated and the photocatalytic reaction was carried out for 5 hours as an accumulated time. As a result, as shown in FIG. 6, when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.295 nm, the Rms of the processed surface after the processing was, 0.179 nm in case of the reaction for 1 hour, 0.136 nm in case of the reaction for 2 hour as an accumulated time, 0.125 nm in case of the reaction for 3 hour as an accumulated time, 0.121 nm in case of the reaction for 4 hour as an accumulated time, and 0.119 nm in case of the reaction for 5 hour as an accumulated time. As just described above, in accordance with the processing method according to Example 1, the flatness of the surface of SiC single crystal substrate can be improved. In addition, when a measurement whether the processing-degenerated layer was generated on the processed surface after the processing or not was carried out, the processing-degenerated layer did not exist.

Also in case that the processing similar to Example 1 was carried out by using KTaO₃, SrTiO₃, ZrO₂, NbO₃, ZnO, WO₃, and SnO₂ as the photocatalyst film 12, the root-mean-square surface roughness (Rms) was improved similarly to Example 1. In particular, in case that the reaction time was controlled to not less than 2 hours, the Rms of the processed surface after the processing could be flattened within a range of 0.100 nm to 0.150 nm in all the cases. It is understood that metal oxides other than TiO₂, if they have a photocatalytic function, can also improve the flatness of the surface of SiC single crystal substrate as the photocatalyst film 12.

Example 2

FIG. 4 is a conceptual view schematically showing an outline of processing method according to Example 2.

In Example 2, the photocatalyst film 12 and the aqueous solution 32 as the processing solution were heated so that a SiC substrate as the workpiece 20 was processed.

In particular, in Example 2, a SiC substrate of a single crystal that is similar to the SiC substrate used in Example 1 was used. Namely, as the SiC substrate used in Example 2, an n-type 4H—SiC that has a diameter of 50 mm and has (0001) Si face inclined by 8 degrees in a [11-20] direction was used. In addition, an electrical resistivity of the SiC substrate was 0.017 Ωcm. In Example 2, similarly, the Si face was used as the processed surface, and as the surface (Si face) to be processed before the processing, a surface to which the CMP treatment was applied after a mechanical mirror-polishing was applied thereto was used.

First, the surface of SiC substrate was cleaned with a 10% HF aqueous solution. Next, a quartz substrate 14 on which the photocatalyst film 12 was formed and the SiC substrate as the workpiece 20 were introduced into a glass beaker 70. In this case, the processed surface of the SiC substrate and the photocatalyst film 12 were brought into contact with each other. Next, an aqueous solution 32 prepared by adding methanol as a radical scavenger to water was introduced into the glass beaker 70 as a processing solution. Here, as the aqueous solution 32, an aqueous solution prepared by adjusting the concentration of methanol to 50% (v/v %) was used. In this case, the aqueous solution 32 was introduced into the glass beaker 70 so that the surface of aqueous solution 32 was located at an upper side than an interface of the photocatalyst film 12 and the processed surface of the SiC substrate (namely, a position that a least the interface exists in the aqueous solution 32). Due to this, the aqueous solution 32 entered the interface of the photocatalyst film 12 and the processed surface of the SiC substrate by capillarity so that an aqueous solution film layer 34 was formed.

Subsequently, an electric current applied to a heater 80 was adjusted by a heater controller 82, and the temperature of aqueous solution 32 was heated up to 60 degrees C. Due to this, both of the aqueous solution 32 as the processing solution and the photocatalyst film 12 was heated to 60 degrees C. Next, an ultraviolet light 62 having an ultraviolet light illuminance adjusted to 8 mW/cm² was irradiated from a side of rear surface 14 a of the quartz substrate 14 by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 was initiated. With regard to a reaction time of the photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 (namely, a reaction time of the active species and the workpiece), the processed surface was observed after every reaction forl hour and the reaction was carried out up to 5 hours as an accumulated time.

Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the SiC substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a 10% HF aqueous solution. And, the processed surface after the processing was observed by the AFM. After that, for further 1 hour, the photocatalytic reaction was similarly carried out, and the oxide film was removed and the processed surface was observed. This was repeated and the photocatalytic reaction was carried out for 5 hours as an accumulated time. As a result, as shown in FIG. 6, when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.497 nm, the Rms of the processed surface after the processing was, 0.210 nm in case of the reaction for 1 hour, 0.145 nm in case of the reaction for 2 hour as an accumulated time, 0.136 nm in case of the reaction for 3 hour as an accumulated time, 0.127 nm in case of the reaction for 4 hour as an accumulated time, and 0.124 nm in case of the reaction for 5 hour as an accumulated time. In addition, an oxidizing speed was calculated based on the thickness of oxide film formed on the processed surface. As a result, it was found that the oxidizing speed in case of heating the aqueous solution 32 as the processing solution was increased than in case of not heating it, so that the processing speed of the processed surface was increased. In addition, it was found that the heating of aqueous solution 32 in the process of processing the processed surface was more effective in enhancing the processing speed than in improving the surface roughness of the processed surface. In addition, when a measurement whether the processing-degenerated layer was generated on the processed surface after the processing or not was carried out, the processing-degenerated layer did not exist.

Example 3

In Example 3, the workpiece 20 was processed nearly in the same way as Example 1 except that a GaN substrate was used as the workpiece 20. In particular, the GaN substrate used in Example 3 is a GaN substrate of a single crystal, and an n-type GaN that has a diameter of 50 mm and has (0001) Ga face was used. Further, the GaN single crystal substrate of {0001} face has polarity, one is a Ga face formed of which outmost face is composed of Ga atoms and another is a N face formed of which outmost face is composed of N atoms. In Example 3, the Ga face was used as the processed surface. In addition, in Example 3, the surface (Ga face) to be processed before the processing is a surface to which the CMP treatment was applied after a mechanical mirror-polishing was applied thereto. Further, in case that the N face was used as the processed surface, although the case is somewhat different in a processing speed from the case that the Ga face was used, a mechanism of advancement of the processing is similar to the case of the Ga face.

Photocatalytic Reaction Type Chemical Processing of GaN Substrate

In particular, the processing method according to Example 3 is as follows. First, the surface of GaN substrate was cleaned with a 10% HF aqueous solution. Next, a quartz substrate 14 on which the photocatalyst film 12 was formed and the GaN substrate were introduced into a glass beaker 70. In this case, the processed surface of the GaN substrate and the photocatalyst film 12 were brought into contact with each other. Next, an aqueous solution 32 prepared by adding methanol as a radical scavenger to water was introduced into the glass beaker 70 as a processing solution. Here, as the aqueous solution 32, an aqueous solution prepared by adjusting the concentration of methanol to 50% (v/v %) was used. In this case, the aqueous solution 32 was introduced into the glass beaker 70 so that the surface of aqueous solution 32 was located at an upper side than an interface of the photocatalyst film 12 and the processed surface of the GaN substrate (namely, a position that at least the interface exists in the aqueous solution 32). Due to this, the aqueous solution 32 entered the interface of the photocatalyst film 12 and the processed surface of the GaN substrate by capillarity so that an aqueous solution film layer 34 was formed.

Next, an ultraviolet light 62 having an ultraviolet light illuminance adjusted to 8 mW/cm² was irradiated from a side of rear surface 14 a of the quartz substrate 14 by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 was initiated. A reaction time of the photocatalytic reaction between the photocatalyst film 12 and the aqueous solution 32 was set to 1 hour.

Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the GaN substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a concentrated sulfuric acid. As a result, when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.261 nm, the Rms of the processed surface after the processing was 0.178 nm. As just described above, in accordance with the processing method according to Example 3, the flatness of the surface of GaN single crystal substrate can be improved. In addition, when a measurement whether the processing-degenerated layer was generated on the processed surface after the processing or not was carried out, the processing-degenerated layer did not exist.

Also in case that the processing similar to Example 3 was carried out by using sapphire, ruby, and diamond as the workpiece 20, although there was difference in the processing speed, the root-mean-square surface roughness (Rms) of the surface of workpiece 20 was improved similarly to Example 3. This shows that even a poor processability substance other than SiC and GaN as the workpiece 20 can be improved on the flatness of surface thereof.

Comparative Example 1

In Comparative Example 1, a SiC substrate was processed under the same conditions that were used in Example 1 except that only water was used as a processing solution instead of using an aqueous solution prepared by adding methanol to water.

The SiC substrate used in Comparative Example 1 was a SiC substrate of a single crystal similarly to Example 1, and an n-type 4H—SiC that has a diameter of 50 mm and has (0001) Si face inclined by 8 degrees in a [11-20] direction was used. In addition, an electrical resistivity of the SiC substrate was 0.017 Ωcm. And, the processed surface was the Si face, and the surface (Si face) to be processed before the processing is a surface to which the CMP treatment was applied after a mechanical mirror polishing.

In the processing method according to the Comparative Example 1, first, the surface of SiC substrate was cleaned with a 10% HF aqueous solution. Next, a quartz substrate 14 on which the photocatalyst film 12 was formed and the SiC substrate were introduced into a glass beaker 70. In this case, the processed surface of the SiC substrate and the photocatalyst film 12 were brought into contact with each other. Next, water as the processing solution was introduced into the glass beaker 70. In this case, water was introduced into the glass beaker 70 so that the surface of water was located at an upper side than an interface of the photocatalyst film 12 and the processed surface of the SiC substrate (namely, a position that at least the interface exists in water). Due to this, water entered the interface of the photocatalyst film 12 and the processed surface of the SiC substrate by capillarity so that a water film layer was formed.

Next, an ultraviolet light 62 having an ultraviolet light illuminance adjusted to 8 mW/cm² was irradiated from a side of rear surface 14 a of the quartz substrate 14 by using a high pressure mercury vapor lamp. Due to this, a photocatalytic reaction between the photocatalyst film 12 and water was initiated. With regard to a reaction time of the photocatalytic reaction between the photocatalyst film 12 and water (namely, a reaction time of the active species and the workpiece), the processed surface was observed after every reaction fort hour and the reaction was carried out up to 5 hours as an accumulated time.

Due to the photocatalytic reaction, an oxide film was formed on the processed surface of the SiC substrate. After the photocatalytic reaction for 1 hour, the oxide film was removed by a 10% HF aqueous solution. And, the processed surface after the processing was observed by an atomic force microscope (AFM). After that, for further 1 hour, the photocatalytic reaction was similarly carried out, and the oxide film was removed and the processed surface was observed. This was repeated and the photocatalytic reaction was carried out for 5 hours as an accumulated time. As a result, as shown in FIG. 6, when a root-mean-square surface roughness (Rms) of the processed surface before the processing was 0.354 nm, the Rms of the processed surface after the processing was, 0.348 nm in case of the reaction for 1 hour, 0.340 nm in case of the reaction for 2 hour as an accumulated time, 0.331 nm in case of the reaction for 3 hour as an accumulated time, 0.335 nm in case of the reaction for 4 hour as an accumulated time, and 0.328 nm in case of the reaction for 5 hour as an accumulated time. In the processing method according to the Comparative Example 1, the flatness of the surface of SiC single crystal substrate was hardly improved. It is considered that the reason why the flatness was hardly improved is that in Comparative Example 1, the diffusion distance of the active species 40 was not controlled, so that the active species 40 reached concave portions in association with progression of the processing and the processing mode was shifted to an isotropic mode. As just described above, the RMS of surface 20 a to be processed of Comparative Example 1 is inferior to the case that the radical scavenger 42 was added.

Control of Diffusion Distance of Active Species

Hereinafter, a method of controlling a diffusion distance of the active species (hydroxyl radicals) in the processing solution in the processing method according to the invention will be explained.

FIG. 5 is a graph showing an oxygen atom concentration in the respective surfaces of SIC substrates of Comparative Example 2 corresponding to a case before the processing method of the invention is applied thereto, Example 1 corresponding to a case that an aqueous solution of water and ethanol is used as a processing solution and Comparative Example 1 corresponding to a case that only water is used as a processing solution.

In particular, first, the oxygen atom concentration in the surface of SiC substrate before the processing method according to Example 1 is applied thereto was measured by Auger electron spectroscopy analysis. In this case, the measurement resulted in measuring the oxygen atom concentration of a natural oxide film in the surface of SiC substrates (hereinafter, referred to as “oxygen concentration of Comparative Example 2”). In addition, the oxygen atom concentration in the surface of SiC substrate to which the processing due to the processing method according to Example 1 was applied was measured by Auger electron spectroscopy analysis (hereinafter, referred to as “oxygen concentration of Example 1”). Further, the oxygen atom concentration in the surface of SiC substrate to which the processing due to the processing method according to Comparative Example 1 was applied was measured by Auger electron spectroscopy analysis (hereinafter, referred to as “oxygen concentration of Comparative Example 1”).

Referring to FIG. 5, the oxygen atom concentration in the surface of SiC substrate was increased in ascending order of the oxygen concentration of Comparative Example 2, the oxygen concentration of Example 1 and the oxygen concentration of Example 1. This shows that the oxidation reaction in the surface of SiC substrate was inhibited due to the fact that methanol as the radical scavenger 42 was added to water. Namely, this shows that the radical scavenger 42 was added to water, so that the diffusion distance of hydroxyl radicals as the active species 40 in water can be controlled. Consequently, it is understood that in Example 1, the oxidation reaction in the surface of the workpiece 20 can be advanced sequentially from a part of the workpiece 20 that has the shortest distance from the surface of photocatalyst film 12, and the diffusion distance of the active species 40 can be controlled due to existing of the radical scavenger 42.

Also in case that the processing was carried out similarly to Example 1 by using ethanol, propanol and butanol as the radical scavenger 42, the oxygen atom concentration in the surface of SiC substrate showed a tendency similar to Example 1. This show that the diffusion distance of hydroxyl radicals as the active species 40 in water can be also controlled due to adding protic organic compounds other than methanol as the radical scavenger 42 to water.

From the above, in accordance with the processing methods according to Examples, the diffusion distance in the processing solution 30 of active species 40 (for example, in case that the main component of processing solution 30 is water, hydroxyl radicals) generated from the processing solution 30 by the photocatalytic action of photocatalyst film 12 can be controlled due to adding the radical scavenger 42 to the processing solution 30. Due to this, the workpiece 20 that has a surface without polishing marks can be provided and simultaneously the surface can be flattened at the atom level.

Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth. 

1. A processing method comprising: disposing a workpiece having a processed surface in a processing solution, disposing a photocatalyst film in the processing solution opposite the processed surface, irradiating the photocatalyst film with a light, so as to generate active species from the processing solution by a photocatalytic action of the photocatalyst film, controlling a diffusion distance of the active species in the processing solution by a radical scavenger added to the processing solution, and chemically reacting the active species with surface atoms of the processed surface and generating a chemical compound to be eluted in the processing solution, so as to process the workpiece.
 2. The processing method according to claim 1, wherein the workpiece is processed by controlling a temperature of at least one member selected from the group consisting of the photocatalyst film, the workpiece and the processing solution.
 3. The processing method according to claim 1, wherein the radical scavenger comprises a protic organic compound.
 4. The processing method according to claim 3 wherein the protic organic compound comprises one of methanol, ethanol, propanol and butanol, or a mixture liquid of not less than two selected therefrom.
 5. The processing method according to claim 1, wherein the photocatalyst film comprises a film of TiO₂, and the TiO₂ comprises an anatase-type crystal or a rutile-type crystal, or a mixed crystal of the anatase-type crystal and the rutile-type crystal.
 6. The processing method according to claim 1, wherein the light has a wavelength of not more than 420 nm.
 7. The processing method according to claim 1, wherein the photocatalyst film is disposed on a substrate formed of quartz or glass.
 8. The processing method according to claim 7 wherein the light is irradiated from a side of the substrate toward the photocatalyst film, so as to generate active species.
 9. The processing method according to claim 1, wherein the workpiece disposed in the processing solution comprises at least one material selected from the group consisting of SiC, GaN, sapphire, ruby and diamond. 